I want to build a 1x12 extension cab for my classic 30. I am wondering a couple things that hopefully someone here can help with. 1.) When i plug into the ext cab plug i am assuming the speaker in the amp will still be active. Is that right? 2.) does adding an ext speaker mean i need to change the speaker in the amp. 3.) the ext plug does not have an ohm rating. i see both 8 and 16 ohm speakers avail. How do I know which one to buy and does it even matter?

Thanks for any info.